The invention relates to a method for conducting flow measurements on flowing media according to the ultrasonic Doppler method. It likewise relates to an apparatus for carrying out such a method.
With the ultrasonic Doppler method, as is known, flow measurements can be conducted in such a fashion that, in the end result, one obtains the flow velocity, the flow cross section, or the volume flow. The volume flow Q of flowing media in the case of a randomly chosen velocity profile, as is known, is given by the relationship Q=F.multidot.v, where F represents the cross-sectional area of the flow and v represents a mean value of the velocity of flow.
The mean value v can be ascertained according to an ultrasonic Doppler method such as is described e.g. in the U.S. Pat. No. 3,675,192. The cross-sectional area can likewise be measured according to the ultrasonic Doppler method, for example, according to the manner of the German AS No. 18 12 017, indirectly via the ascertainment of a vessel radius. Other measurement possibilities beyond the ultrasonic Doppler method are likewise applicable. All known methods for the determination of the flow velocity v and the cross-sectional area F, aside from the fact that some methods (in particular, according to German AS No. 18 12 017) are relatively imprecise and therefore subject to measurement errors, have the disadvantage of dependency on the irradiation angle of the ultrasound into the flowing medium. This problem could be confronted up to the present time only, at least as far as the measurement of the flow velocity is concerned, in that either the angle was explicitly measured with additional technical outlay (e.g. U.S. Pat. Nos. 3,974,692 or 4,127,842) or, by means of a two-beam intersection measurement (e.g. according to the U.S. Pat. No. 3,766,517), the angle was eliminated. However, an angle-independent cross-sectional measurement could thus still not yet be conducted.
As far as the volume flow Q is concerned, the latter can, of course, be ascertained in an angle-independent fashion according to such a Doppler method as is described e.g. in the article "An Ultrasonic Technique for Unambiguous Measurement of Blood Volume Flow" by C. F. Hottinger and J. D. Meindl in the Journal "Ultrasonics Symposium Proceedings", IEEE Cat. No. 74 CHO 896-ISU, 1974. According to this method, the volume flow Q results according the equation: ##EQU1## whereby P.sub.m1 represents the measured power of the Doppler signals from a first Doppler signal collecting region with the volume V.sub.m1 and the thickness d, and P.sub.p represents the measured Doppler signal power from a second Doppler signal collecting region with the volume V.sub.p. The flow velocity v is, indeed, associated with the cosine of the irradiation angle .alpha. of the ultrasound into the flowing medium. Since the quantity v.multidot.cos .alpha., however, is directly measured as a velocity component in the sound propagation direction, one finds the volume flow Q without additional angular measurement. A similar method for the measurement of the volume flow Q according to the ultrasonic Doppler method is described in U.S. Pat. No. 3,977,247. Here the volume flow Q results independently of the irradiation angle of the ultrasound into the flowing medium during measurement on flow vessels running in a curved fashion since, in such an instance, the measurement arrangement can be so adjusted that the irradiation angle relative to the flow direction is virtually zero. However, in the case of any other irradiation angle the volume flow results solely according to the relation: ##EQU2## where P.sub.p again represents the power of the Doppler signals from the second Doppler signal collecting region with the volume V.sub.p, and P.sub.m2 represents the power of Doppler signals which originate from a third Doppler signal collecting region with the width b.
According to the method of the article of C. F. Hottinger and J. D. Meindl, as well as according to the exceptional case of the U.S. Pat. No. 3,977,247, the volume flow can, indeed, be directly, angle-independently measured; however, if one simultaneously also wishes to ascertain the flow velocity v and/or the flow cross-section F, then, even in the case of the method of the article of C. F. Hottinger and J. D. Meindl, an angle-dependent measurement must be conducted, since the blood flow velocity, or the cross-sectional area of the flow, respectively, are dependent upon the irradiation angle .alpha. according to a cosine relation. Regarding the general case of the U.S. Pat. No. 3,977,247, it has already been mentioned that the measurement of the volume flow Q is only possible when simultaneously also the flow velocity v in combination with sin .alpha. is measured.